A hydraulic or pneumatic cylinder, having an actuating rod extending from one end, is known as a single-ended cylinder, with the end of the cylinder having the rod referred to as the rod end, and the opposite end referred to as the blind end. Due to the presence of the rod attached to the cylinder piston, the volume for hydraulic fluid differs on either side of the piston. More importantly, the rate of change of volume, or the flow of hydraulic fluid, differs as the piston is moved in either direction. In a system where the cylinder is directly connected to a reversible pump, when the cylinder is being actuated to extend the rod, the fluid flowing from the rod-end of the cylinder is less than that required to be supplied by the pump to the blind end of the cylinder, for a particular rate of extension of the cylinder. Typically, the pump draws additional working fluid from a reservoir to supply this excess fluid, often through the same path that originally supplied working fluid to the pump, to initially fill the system with the working fluid. This may be a one-way valve between the pump and the reservoir.
When the cylinder is actuated to retract the rod, a greater flow of fluid leaves the blind end of the cylinder than is being supplied to the rod end of the cylinder. This excess return flow pressurizes the inlet of the pump. This excess pressure is known as superchange pressure. Since a pump has inherent leakage, this inherent leakage may be relied on to relieve the excess pressure at the inlet of the pump, passing this additional fluid to the reservoir through the pump's drain outlet. However, in a well-made, energy efficient pump, the inherent leakage is low, so that a relief valve is provided at the inlet or outlet of the pump to return excess fluid to the reservoir. This wastes energy in converting supercharge pressure to heat in the relief valve, and in slowing cylinder rod retraction. This is most important in mobile applications, such as trim and tilt units for outboard motors on small boats, where the power is supplied from an intermittent duty motor, where high electrical currents are involved, and heating and increased electrical resistance losses result from increased operation time.
In more complex and expensive systems, a constant-pressure pump is used, and the direction of cylinder rod travel is determined by a separate reversing valve connected to the cylinder, and to the pump and reservoir. In such a system, any excess fluid is passed to, or pulled from, the reservoir through the reversing valve. However, a constant pressure pump operates by generating a higher pressure than desired, and regulating the pressure down to the desired valve by dissipating the excess pressure in a back-pressure or relief valve. This type of system is capable of providing substantially equal speeds of extension and retraction of a cylinder's actuating rod, since the pump flow is whatever is necessary to maintain the predetermined pressure. However, such a system, besides requiring additional heavy and expensive components, is unsuitable where energy consumption is important, due to the energy lost in the regulating valve, converting excess pump pressure to heat, and the decreased efficiency of a higher pressure pump.
Applicant's invention overcomes these and other deficiencies of known hydraulic systems.